Users of computing devices (e.g., laptops, cellular phones, and personal digital assistants) often need to communicate in real time. A common form of real-time communications is provided by instant messaging services. An instant messaging service allows a participant at an endpoint to send messages and have them received within a second or two by the other participant in a conversation. The receiving participant can then send responsive messages to the other participant in a similar manner.
To support real-time communications, communications applications typically need to establish and manage connections (also referred to as sessions or dialogs) between computing devices. A session is a set of interactions between computing devices that occurs over a period of time. As an example, instant messaging services require a networking protocol to establish and manage communications between participants. These services may use various mechanisms to establish sessions, such as a “Session Initiation Protocol” (“SIP”). SIP is an application-level control protocol that computing devices can use to discover one another and to establish, modify, and terminate sessions between computing devices. SIP is a proposed Internet standard. The SIP specification, “RFC 3261,” is available at <http://www.ietf.org/rfc/rfc3261.txt>.
A SIP network comprises entities that can participate in a dialog as an endpoint, server, or both. SIP supports four types of entities: user agent, proxy server, redirect server, and registrar. User agents initiate and terminate sessions by exchanging messages with other SIP entities. A user agent can be a user agent client, which is generally a computer system that initiates SIP requests, or a user agent server, which is a computer system that generally receives SIP requests and responds to such requests. As examples, “IP-telephones,” personal digital assistants, and various other types of computing systems may be user agents. A computer system can be a user agent client in one dialog and a user agent server in another, or may change roles during the dialog. A proxy server is an entity that acts as a server to clients and a client to servers. In so doing, proxy servers intercept, interpret, or forward messages between clients and servers. A redirect server accepts a SIP request and generates a response directing the client that sent the request to contact an alternate network resource. A registrar is a server that accepts registration information from SIP clients and informs a location service of the received registration information.
SIP supports two message types: requests, which are sent from a client to a server, and responses, which are sent from a server to a client, generally when responding to a request. A SIP message is comprised of three parts. The first part of a SIP message is a “start line,” which includes fields to indicate a message type and a protocol version. The second part of a SIP message comprises header fields whose values are represented as name-value pairs. The third part of a SIP message is the message's body, which is used to describe the session to be initiated or contain data that relates to the session. Message bodies may appear in requests or responses.
To start a conversation, an inviting participant typically sends an invitation from an endpoint to the instant messaging service, which then forwards the invitation to an endpoint for the receiving participant. The invitation can take place using the SIP INVITE protocol. The endpoint for the receiving participant then accepts the invitation, forming a conversation between the two participants which is typically represented as a conversation window at each participant's instant messaging endpoint. The participants can then each type messages back and forth to each other until the conversation is terminated by either of the participants by, for example, closing the conversation window.
It is becoming increasingly common for multiple participants to be involved in a conversation. For example, today's businesses often assign business projects to work groups with members physically located in disparate locations, and these members need to be involved in the same conversation. However, because typical instant messaging services rely on a point-to-point protocol, such as SIP, a conversation is limited to the two participants who are engaged in an instant messaging session. If another, third participant who wants to join the existing conversation establishes an instant messaging session with either one of the two participants who are currently engaged in the existing conversation, typical instant messaging services do not provide a mechanism for indicating or associating the newly created instant messaging session with the existing conversation. Rather, two distinct conversations, each involving two of the three participants, are created.